An aerosol is defined as an assembly of liquid or solid particles suspended in a gaseous medium. (See Aerosol Measurement, Willeke and Baron, Wiley-Interscience 1993.) It is known that aerosols of appropriate particle size, can be used to deliver drugs to organs and tissues such as the lung and mucosa. (See Gonda, I., “Particle Deposition in the Human Respiratory Tract,” The Lung: Scientific Foundations, 2nd ed., Crystal, West, et al. editors, Lippincott-Raven Publishers, 1997).
A problem in generating an aerosol is maintaining the purity of a compound being administered into the lung, as an aerosol. This is a critical issue that must be addressed before inhalation delivery of a compound to humans will be acceptable to regulatory agencies, physicians and patients. Any compound administered to humans must meet strict purity requirements regulated by government agencies and industry. For example, the United States Food and Drug Administration mandates purity requirements for pharmaceutical materials sold in the United States to protect the health of consumers of those products. Purity requirements are often material specific. Maximum impurity or degradant levels are specified at the time of manufacture of compounds as well as at the time of their consumption or administration. Any aerosolization device or process that will be utilized for pharmaceutical applications, therefore, must deliver materials meeting purity requirements. Mechanisms of chemical degradation that might occur during vaporization and aerosolization, the processes relevant to this invention, are discussed below.
Currently approved products for inhalation administration of physiologically acting compounds can be divided into several categories: dry powder inhalers, nebulizers, and pressurized metered dose inhalers. The desired particle size of these methods and devices usually are in the fine aerosol region (1-3 micron) and not in the ultra fine region (10-100 nm). A large percentage of these devices fall short of the type of particle size control desirable for reproducible and efficient delivery of compounds to the lung. Additionally current devices focus on the fine aerosol region because to date a practical device that can reproducibly generate an ultra fine aerosol has not been commercially available for drug delivery to the lung.
There are many types of dry powder inhalers (DPI's) on the market with some common problems. The first problem is the manufacturing of the dry powder. For a dry powder inhalation system it is necessary to mill the drug until it falls into the desirable particle range. Some mills used for micronization are known to produce heat, which can cause degradation of the drug, and tend to shed metallic particles as contaminants. Following milling it is often necessary to mix the drug with a carrier to impart flowability. The micronized drug and the drug-excipient mix must be maintained in a dry atmosphere lest moisture cause agglomeration of the drug into larger particles. Additionally it is well known that many dry powders grow as they are delivered to the patient's airways due to the high levels of moisture present in the lung. Thus, this approach requires scrupulous attention during milling, blending, powder flow, filling and even administration to assure that the patient receives the proper particle size distribution.
Nebulizers generate an aerosol from a liquid, some by breakup of a liquid jet and some by ultrasonic vibration of the liquid with or without a nozzle. All liquid aerosol devices must overcome the problems associated with formulation of the compound into a stable liquid state. Liquid formulations must be prepared and stored under aseptic or sterile conditions since they can harbor microorganisms. This necessitates the use of preservatives or unit dose packaging. Additionally solvents, detergents and other agents are used to stabilize the drug formulation. The FDA is increasingly concerned about airway hypersensitivity from these agents.
Pressurized metered dose inhalers, or pMDI's, are an additional class of aerosol dispensing devices. PMDI's package the compound in a canister under pressure with a solvent and propellant mixture, usually chlorofluorocarbons (CFC's, which are being phased out due to environmental concerns), or hydroflouroalkanes (HFA's). Upon being dispensed a jet of the mixture is ejected through a valve and nozzle and the propellant “flashes off” leaving an aerosol of the compound. With pMDI's particle size is hard to control and has poor reproducibility leading to uneven and unpredictable bioavailability. pMDIs are inefficient because a portion of the dose is lost on the walls of the actuator, and due to the high speed ejection of the aerosol from the nozzle, much of the drug impacts ballistically on the tongue, mouth and throat and never gets to the lung.
Another method suggested in the prior art to generate aerosols is to volatilize the drug and administer the vapor to a patient. (See Rosen, PCT Publication No. 94/09842, published May 11, 1994.) However, the teaching of Rosen is not a viable solution to the problem because it yields (1) a large quantity of degradation products, and (2) too much variability in particle size distribution (PSD) to insure reproducible and predictable bioavailability.
Predicting the reactions that result in a compound's degradation, and anticipating the energies necessary to activate those reactions are typically very difficult. Reactions may involve only the parent compound or may involve other chemicals such as oxygen in air and materials in the surfaces to which the compound may be exposed. Reactions may be single step or multiple steps, leading to the potential of many degradation products. Activation energies of these reactions depend on molecular structures, energy transfer mechanisms, transitory configurations of the reacting molecular complexes, and the effects of neighboring molecules. Frequently, on the practical macroscopic scale, a drug dose may suffer from many degradation reactions in progress at the same time. Because of this complex potential for degradation, drug substances are often stored at or below room temperature. International health authorities recommend that the stability of a drug be evaluated under exaggerated (stress) conditions to determine the mechanism of degradation and the degradant structures. (See Guidance for Industry: Stability testing of drug substances and products; FDA CDER May 27, 1998). For these tests, 50° C. is recognized as a stress temperature.
The present invention overcomes the foregoing disadvantages and problems, making it possible to produce pure aerosols of degradable compounds wherein the particle size is stable and selectable.